The invention relates to environmental control systems. More specifically, the invention relates to an environmental control system including redundant air cycle machines.
An environmental control system for an aircraft is designed to control airflow into the aircraft's passenger cabin as well as air temperature inside the passenger cabin. Most aircraft air environmental control systems operate on an air cycle refrigeration principle. Compressed air is obtained from a compressor stage of the aircraft's main engine, cooled with ambient air to near-ambient temperature in an air-to-air heat exchanger and then expanded in an air cycle machine to provide a stream of cooled, conditioned air. The conditioned air is supplied to the passenger cabin. Although somewhat expanded, the conditioned air also pressurizes the cabin.
There will be occasions when the air cycle machine fails. When failure occurs in corporate aircraft, hot bleed air from an air-to-air heat exchanger upstream the air cycle machine is typically provided to the cabin. The cabin remains pressurized, but the hot bleed air makes it extremely uncomfortable for those passengers inside the cabin. Instead of dumping the bleed air into the cabin, manufacturers of larger commercial aircraft have opted for redundant air cycle machines in separate packages. If one package fails, the redundant package provides backup and keeps the passengers comfortable.
However, redundancy comes at a cost. Adding a second air-to-air heat exchanger, a second water extractor, a second condenser, a second reheater and a second air cycle machine all increase the parts count and weight of the aircraft. Increasing the parts count results in a decrease in reliability. Increasing the weight results in an increase in fuel consumption and, therefore, the cost of operating the aircraft.